Upon transmitting speech/audio signals in, for example, a packet communication system represented by Internet communication and mobile communication system, compression/coding techniques are often used to improve the efficiency of transmission of speech/audio signals (i.e. music signals). Also, recently, there is a growing need for techniques of simply encoding speech/audio signals at a low bit rate and encoding speech/audio signals of a wider band.
To meet this need, there is a technique for encoding signals of a wide frequency band at a low bit rate (e.g. see Patent Document 1). According to this technique, the overall bit rate is reduced by dividing an input signal into the lower-band signal and the higher-band signal and by encoding the input spectrum replacing the spectrum of the higher-band signal with the spectrum of the lower-band signal.    Patent Document 1: Japanese Translation of PCT Application Laid-Open No. 2001-521648